White LED strings are widely used as backlight of liquid crystal displays (“LCDs”) in computers, televisions, and other electronic devices. Typically, an LED string is powered by a switch-mode driver system. A primary switch device is controlled by a feedback signal which represents the current flowing through the LED string. The term “primary switch” as used herein generally refers to a primary side switch in an isolated converter and to a high-side switch in a non-isolated converter such as a buck converter.
For regulating brightness of an LED string, another switch device is coupled in series with the LED string to function as a dimming switch. FIG. 1 illustrates an operational waveform of a conventional LED driver. The main circuit provides a constant current to the LED string, and the primary switch device is controlled by a pulse width modulation (“PWM”) drive signal. As shown in FIG. 1, the PWM dimming signal, as the gate signal of the dimming switch device, regulates the brightness of the LED string by varying the duty cycle. The frequency of the PWM drive signal is higher than that of the PWM dimming signal. When the PWM dimming signal is in a high-level, the dimming switch turns on, and current flows through the LED string. As a result, a signal ILED by which the PWM dimming signal is modulated is in a high level as well. When the PWM dimming signal is in a low-level, the dimming switch turns off, and a current no longer flows through the LED string corresponding to a zero PWM driver signal.